IssueTracker

A few notes:

1. Should be using viewLifecyclerOwner.lifecycleScope instead of just lifecycleScope in a Fragment.
2. Cancellation is happening as a result of the fragment automatically paging over when they are initially created / added to viewpager2 (this is expected as far as I can tell)
3. Right now RemoteMediator kind of acts as a dumb callback whenever PagingSource runs out of items to load, so REFRESH error doesn't prevent remote PREPEND / APPEND. It's not clear to me yet whether it should or if we should change guidance to just handle APPEND. This is due to us changing the behavior of Remote REFRESH after the codelab was produced, so I'm very sorry for that. REFRESH is meant to handle any init that needs to happen, technically the initial network page fetch happens as a result of running out of items on the APPEND side.

e.g., Here is a modified sample from the opening post / #1's repro project that fixes the issue:

PageKeyedRemoteMediator.kt

override suspend fun load(
    loadType: LoadType,
    state: PagingState<Int, RedditPost>
): MediatorResult {
    try {
        // Get the closest item from PagingState that we want to load data around.
        val loadKey = when (loadType) {
            REFRESH -> {
                db.withTransaction {
                    postDao.deleteBySubreddit(subredditName)
                    remoteKeyDao.deleteBySubreddit(subredditName)
                }

                return MediatorResult.Success(endOfPaginationReached = false)
            }
            PREPEND -> return MediatorResult.Success(endOfPaginationReached = true)
            APPEND -> {
                // Query DB for SubredditRemoteKey for the subreddit.
                // SubredditRemoteKey is a wrapper object we use to keep track of page keys we
                // receive from the Reddit API to fetch the next or previous page.
                val remoteKey = db.withTransaction {
                    remoteKeyDao.remoteKeyByPost(subredditName)
                }

                when {
                    remoteKey == null -> null
                    // We must explicitly check if the page key is null after initial load, since the
                    // Reddit API informs the end of the list by returning null for page key, but
                    // passing a null key to Reddit API will fetch the initial page.
                    remoteKey.nextPageKey == null -> {
                        return MediatorResult.Success(endOfPaginationReached = true)
                    }
                    else -> remoteKey.nextPageKey
                }
            }
        }

        val data = redditApi.getTop(
            subreddit = subredditName,
            after = loadKey,
            before = null,
            limit = state.config.pageSize * 3
        ).data

        val items = data.children.map { it.data }

        db.withTransaction {
            remoteKeyDao.insert(SubredditRemoteKey(subredditName, data.after))
            postDao.insertAll(items)
        }

        return MediatorResult.Success(endOfPaginationReached = items.isEmpty())
    } catch (e: IOException) {
        return MediatorResult.Error(e)
    } catch (e: HttpException) {
        return MediatorResult.Error(e)
    } catch (e: CancellationException) {
        Log.d("RemoteMediator","$subredditName $loadType canceled")
        throw e
    }
}

Will keep this bug open to track the decision on blocking RemoteMediator PREPEND / APPEND after remote REFRESH error and any possible updates to the Codelab / Github samples.

The inconvenience of blocking on REFRESH is going to surface via needing to call retry(), but it's possible there's something better we can do to handle cancellation more gracefully since we don't want to keep creating new PagingData and also support loading in the background. (the thing to change to control this is when you start collecting from Pager.flow, and the scope collection happens in).

I have a question about the modified sample. Shouldn't deleteByXXX be moved to after redditApi execute successfully?

The UI side will see blink twice in this sample: Show db cache first then disappeared, show data again after redditApi successful and insert data. The user experience is pretty bad.

To be clear the above sample is a workaround a deeper issue in Paging which isn't handling cancellation gracefully due to Remote REFRESH errors not blocking remote PREPEND / APPEND.

I found that if using fragment.lifecycleScope.launchWhenResume to collect each page data then it will work normally. ViewPager2 FragmentStatePager will move fragment to onResume/onPause.

Good to hear, I think we can still do a better job of handling cancellation and propagating remote state, e.g., if you have many pages in viewpager and flip back and forth I think you'll still eventually get stuck, since the collector on PagingData will get canceled and there's no invalidate signal to restart it without manually calling adapter.refresh().